The present invention relates to a drive circuit for driving a group of driven elements such as, for example, an array of light emitting diodes (LEDs) disposed in an electro-photography printer as a light source, an array of heating resistors disposed in a thermal printer, and an array of display units disposed in a display device. The present invention also relates to a light emitting diode (LED) head including the drive circuit; and an image forming apparatus including the light emitting diode (LED) head.
In the specification, a light emitting diode may be referred to as an LED; a monolithic integrated circuit may be referred to as an IC; an n-channel MOS (Metal Oxide Semiconductor) transistor may be referred to as an NMOS (transistor); and a p-channel MOS transistor may be referred to as a PMOS (transistor).
Further, a high signal level may be referred to as a logical value of one (1), and a low signal level may be referred to as a logical value of zero (0), regardless of a positive logic or a negative logic. When it is necessary to differentiate the positive logic and the negative logic in a logical signal, “−P” may be added to an end of a positive logical signal, and “−N” may be added to an end of a negative logical signal.
In the following description, a group of driven elements is an array of LEDs used in an electro-photography printer as an example.
In a conventional electro-photography printer, a light source selectively irradiates a photosensitive drum charged according to print information, thereby forming a static latent image on the photosensitive drum. Then, toner is attached to the static latent image to form a toner image. Afterward, the toner image is transferred to a sheet, so that the toner image is fixed to the sheet. The light source may be formed of LEDs.
In the conventional electro-photography printer, an LED head is formed of an LED array chip and a driver IC for driving the LED array chip.
The LED head includes a reference voltage generation circuit for generating a reference voltage, so that a drive current for driving LED elements is determined based on the reference voltage generated from the reference voltage generation circuit and a resistor disposed in the driver IC. The resistor is produced through a semiconductor process technology. In general, the resistor is formed of poly-silicon or an impurity diffused resistor, and is integrated in the driver IC in a form of monolithic.
FIG. 13 is a block diagram showing an LED head 19 and a print control unit 1 of the conventional electro-photography printer. As shown in FIG. 13, the electro-photography printer includes the print control unit 1; the LED head 19; and a connection cable 47 connecting the print control unit 1 and the LED head 19. The print control unit 1 is formed of a microprocessor, an RAM, an ROM, an input-output port, a timer, and the likes. The print control unit 1 is disposed in a printing unit of the electro-photography printer for controlling a printing operation according to a control signal from an upper controller.
In the conventional electro-photography printer, the print control unit 1 is usually arranged away from the LED head 19. Accordingly, it is necessary to connect the print control unit 1 and the LED head 19 with the connection cable 47 having a large cable length. In general, the connection cable 47 has a cable length of about 50 cm. When the conventional electro-photography printer is a tandem type color printer having a plurality of photosensitive drums arranged in parallel, the connection cable 47 tends to have a cable length of more than 1 m, thereby causing a problem (described later).
In the following description, as an example, the LED head 19 is capable of printing on a sheet with A-4 size at a resolution of 600 dots per one inch. In this case, the LED head 19 includes a total of 4992 dots of the LED elements. More specifically, the LED head 19 includes 26 of LED arrays, and each LED array is formed of 192 of the LED elements.
As shown in FIG. 13, the LED head 19 includes LED arrays CHP1 and CHP26, and LED arrays CHP3 to CHP24 are omitted in FIG. 3. Driver ICs IC1 and IC 26 are arranged to correspond to the LED arrays CHP1 and CHP26 for driving the LED arrays CHP1 and CHP26, respectively. The driver ICs IC1 and IC 26 are formed of an identical circuit, and adjacent driver ICs are connected in a cascade connection.
The LED array CHP1 includes LED elements LED1 to LED192, so that 192 of the LED elements are arranged per each LED array. Accordingly, the LED array CHP25 includes LED elements LED4609 to LED4800, and the LED array CHP26 includes LED elements LED4801 to LED4992.
In the LED head 19 shown in FIG. 13, 26 of the LED arrays (CHP1 to CHP26) and 26 of the driver ICs (IC1 to IC26) for driving the LED arrays are arranged on a print circuit board (not shown) to face each other. One chip of the driver IC is capable of driving 192 of the LED elements, and 26 chips of the driver ICs are connected in a cascade connection for transmitting in serial print data input from outside.
The LED head shown in FIG. 13 is formed of a semiconductor compound such as GaAsP and AlGaAs, and a forward voltage of each LED is about 1.6 V upon driving.
In the LED head 19 shown in FIG. 13, each of the driver ICs (IC1 to IC26) is formed of an identical circuit, and adjacent driver ICs are connected in a cascade connection. Each of the driver ICs includes a shift resister circuit 44 for receiving a clock signal HD-CLK and performing shift transfer of print data; a latch circuit 43 for latching an output signal of the shift resister circuit 44 according to a latch signal (referred to as HD-LOAD); an AND circuit 42 for receiving outputs of the latch circuit 43 and an inverter circuit 41 to obtain a logic product; an LED drive circuit 40 for supplying a drive current from a power source VDD to the LED element (CHP1 etc.) according to an output signal of the AND circuit 42; and a control voltage generation circuit 45 for generating a control voltage such that the drive current of the LED drive circuit 40 becomes constant.
A strobe signal HD-STB-N is input to the inverter circuit 41. Further, a reference voltage generation circuit 46 is provided, in which a power source terminal thereof is connected to the power source VDD, a ground terminal thereof is connected to the LED head 19, and an output terminal thereof is connected to the control voltage generation circuit 45 of each of the driver ICS IC1 to IC26 for supplying a reference voltage Vref.
Note that the print control unit 1 sends the print data signal HD-DATA, the clock signal HD-CLK, the latch signal HD-LOAD, and the strobe signal HD-STB-N. The connection cable 47 include cables of the control signals (the print data signal HD-DATA, the clock signal HD-CLK, the latch signal HD-LOAD, and the strobe signal HD-STB-N), the power source VDD, and a ground VSS.
FIG. 14 is a circuit diagram showing an LED drive circuit of the driver IC in the LED head 19 of the conventional electro-photography printer. FIG. 14 shows a connection relationship of the LED drive circuit and a peripheral portion thereof, and dot 1 (a peripheral portion the drive circuit of the LED 1) is shown as an example. In FIG. 14, an area 71 surrounded with a hidden line represents the driver IC, and an area 72 corresponds to the LED array.
As shown in FIG. 14, the LED drive circuit includes the inverter circuit 41 shown in FIG. 13, an AND circuit 42, and a latch circuit 51 corresponding to one element of the latch circuit 43 shown in FIG. 13. The latch circuit 51 has a input terminal D connected to an output terminal of a shift register (not shown, corresponding to the shift resister circuit 44 in FIG. 13); an input terminal G connected to the latch signal HD-LOAD; and an output terminal Q connected to one of input terminals of the AND circuit 42.
The inverter circuit 52 is formed of a PMOS transistor 53 and an NMOS transistor 54. A source terminal of the PMOS transistor 53 is connected to the power source VDD. Drain terminals and gate terminals of the PMOS transistor 53 and the NMOS transistor 54 are connected to with each other. A source terminal of the NMOS transistor 54 is connected to an output terminal of an operational amplifier 61 (described later), so that a potential Vcont is applied to the source terminal of the NMOS transistor 54.
A PMOS transistor 55 is also provided. A gate terminal of the PMOS transistor 55 is connected to the drain terminals of the PMOS transistor 53 and the NMOS transistor 54. The LED element LED1 is also provided.
The operational amplifier 61 has an output voltage as the potential Vcont. A resistor 63 has a resistivity of Rref. A PMOS transistor 62 has a gate length same as that of the PMOS transistor 55. The reference voltage generation circuit 46 shown in FIG. 13 generates the reference voltage Vref connected to an inverse input terminal of the operational amplifier 61.
A source terminal of the PMOS transistor 62 is connected to the ground, a gate terminal thereof is connected to the output terminal of the operational amplifier 61, and a drain terminal thereof is connected to one end portion of the resistor 63 and a non-reverse input terminal of the operational amplifier 61. A feedback circuit is formed of the operational amplifier 61, the PMOS transistor 62, and the resistor 63, so that a current flowing through the resistor 63, that is, a current flowing through the PMOS transistor 62, is determined only by the reference voltage Vref and the resistivity Rref of the resistor 63 regardless of a power voltage of the power source VDD.
When the NMOS transistor 54 is turned on, the PMOS transistor 53 becomes an off state. The PMOS transistor 55 has a gate potential same as that of the Vcont potential. Accordingly, the PMOS transistor 55 has a gate-source voltage same as that of the PMOS transistor 62, thereby constituting a current-mirror relationship. As a result, it is possible to adjust the drain current of the PMOS transistor 55 according to the reference voltage Vref, thereby controlling the drive current of the LED element in the LED array 72 at a specific level.
FIG. 15 is a circuit diagram showing the LED drive circuit and the print control unit 1 of the conventional electro-photography printer. In FIG. 15, the output signals and the likes inside the print control unit 1 are omitted, and only the power source VDD is shown. In the connection cable 47, the control signals and the likes are omitted, and only the power source VDD and the ground VSS are shown. The ground VSS has a resistivity of Rg. As shown in FIG. 15, in the conventional electro-photography printer, the LED array 72 includes a ground route shared with that of the driver IC 71.
Patent Reference has disclosed a method of driving an LED element. In Patent Reference, only a principle of the method of driving the LED element has been shown, and no specific circuit diagram has been disclosed. FIGS. 16(a) to 16(c) are circuit diagrams showing the method of driving the LED element. Note that FIG. 16(a) is an equivalent circuit diagram showing a first drive circuit and corresponding to the circuit diagram shown in FIG. 14.
Patent Reference: Japanese Patent Publication No. 08-4153
As shown in FIGS. 16(a) to 16(c), the drive circuit includes a constant current source 81, an LED 82, and a capacity 83, i.e., a model of a junction capacity between an anode terminal and a cathode terminal of the LED 82 and a floating capacity of a wiring. When a switch unit 84 is turned to a side A, the LED 82 is turned off, and when the switch unit 84 is turned to a side B, the LED 82 is turned on.
When the switch unit 84 is turned to the side B and the LED 82 is turned on, a forward voltage VF of the LED 82 (in this case, about 1.6 V) is applied to a capacity Cj between the anode terminal and the cathode terminal of the LED 82.
When the switch unit 84 is switched from the side B to the side A to turn off the LED 82, the drive current from the constant current source 81 is disconnected immediately after the switch unit 84 is switched. Charges accumulated in the capacity Cj are discharged slowly in a forward direction of the LED 82, thereby increasing a switching time.
FIG. 16(b) is an equivalent circuit diagram showing a second drive circuit. In the equivalent circuit diagram shown in FIG. 16(b), when he switch unit 84 is turned to the side B to turn on the LED 82, an operation thereof is the same as that in the first drive circuit shown in FIG. 16(a). When he switch unit 84 is turned to the side A to turn off the LED 82, the anode terminal and the cathode terminal of the LED 82 are short-circuited, thereby discharging charges accumulated in the capacity Cj.
FIG. 16(c) is an equivalent circuit diagram showing a third drive circuit. In the equivalent circuit diagram shown in FIG. 16(c), when he switch unit 84 is turned to the side B to turn on the LED 82, an operation thereof is the same as that in the first drive circuit shown in FIG. 16(a). When he switch unit 84 is turned to the side A to turn off the LED 82, a voltage V is applied between the anode terminal and the cathode terminal of the LED 82. In this case, it is set such that the voltage V is smaller than the forward voltage VF of the LED 82. Accordingly, while charges accumulated in the capacity Cj between the anode terminal and the cathode terminal of the LED 82 are rapidly discharged below the forward voltage VF, the voltage does not become zero and is maintained at the voltage V for a next operation.
FIG. 17 is a graph showing drive waves of the LED drive circuits shown in FIGS. 16(a) to 16(c) of the conventional electro-photography printer. As shown in FIG. 17, the strobe signal HD-STB-N (negative logic) sent to the LED head 19 shown in FIG. 13 shows changes of the LED from the off state to the on state, and then from the on state to the off state.
FIG. 17 shows a voltage Vo between the anode terminal and the cathode terminal of the LED 82 and a luminescent output Po of the LED. When the LED is turned on, the voltage Vo becomes a voltage Vf. The voltage Vo changes differently when the LED is turned off according to the LED drive circuits shown in FIGS. 16(a) to 16(c).
In the LED drive circuit shown in FIGS. 16(a), the voltage Vo changes along a hidden line decreasing gradually accompanied with the capacity Cj discharging gradually. In the LED drive circuit shown in FIG. 16(b), the voltage Vo changes along a solid line. That is, the voltage Vo becomes zero immediately after the strobe signal HD-STB-N is turned off, and the luminescent output Po decreases rapidly as indicated with a solid line.
In the LED drive circuit shown in FIG. 16(c), the voltage Vo is maintained at the voltage V along a projected line. As described above, it is set such that the voltage V is smaller than the forward voltage VF of the LED, thereby flowing no drive current. Further, when the LED is turned on, the voltage Vo changes from the voltage V, so that the luminescent output Po increases in a shorter period of time. When the LED is turned off, similar to the second drive circuit shown in FIG. 16(b), the luminescent output Po decreases rapidly as indicated with the solid line.
Accordingly, in the third drive circuit shown in FIG. 16(c), the luminescent output Po increases and decreases more rapidly than the first and second drive circuits shown in FIG. 16(a) and FIG. 16(b), thereby increasing an operational speed.
In the LED drive circuit of the conventional electro-photography printer shown in FIG. 14, it is difficult to increase the luminescent output in a short period of time, thereby making a fast switching operation difficult. The LED drive circuit of the conventional electro-photography printer shown in FIG. 14 corresponds to the LED drive circuit shown in FIG. 16(a). When the LED is turned on, the drive current is supplied to the drive circuit, and when the LED is turned off, the drive current is disconnected to be in an open state.
Accordingly, immediately after the LED is turned on, a remaining voltage is generated due to charges accumulated in the capacity between the anode terminal and the cathode terminal of the LED. As a result, the discharge current continues to flow through the LED, thereby slowing a response of the LED.
In view of the problems described above, an object of the present invention is to provide a drive circuit capable of solving the problems of the conventional drive circuit. In the drive circuit, it is possible to achieve a fast response of a luminescent output of an LED when the LED is turned off, thereby obtaining a fast operation of the drive circuit and an LED head.
Further objects and advantages of the invention will be apparent from the following description of the invention.